Help needed in formulating an EarthCache

[+barefootjeff]

Quoting from another thread, as this is off-topic for that one:

 

11 hours ago, Neos2 said:

And as always, I encourage people to come in here are ask us to help them come up with ways to develop their EarthCaches.

 

There's a location near home in Brisbane Water National Park called (locally at least) Spion Kop, where I once wanted to put a physical cache but the ranger wouldn't allow it on account of the unofficial (but well-maintained) walking track out there passing near an Aboriginal site. She said, though, that because virtual caches and EarthCaches don't need formal approval under the NSW National Parks Geocaching Policy, one of those would be fine. I ended up picking another nice spot in the park for my physical cache (GC752YF on Scopas Peak) but have kept Spion Kop in mind as a possible virtual cache if I'm ever successful in a Virtual Rewards draw, or for an EC if I can figure out the geology going on there. I'm an electronics engineer by profession (now retired) and the only geology I've studied was in high school over 50 years ago and from the ECs I've done in recent years, so I really don't know much about the subject beyond what I can find online.

 

The main geological feature at Spion Kop is this large rock finger like a dragon's head poking out over the cliff edge:

 

 

Here's a view from a slightly different angle that shows a bit more of the rock:

 

 

It's not a one-off formation, though, as just around the headland is another smaller version (what I call Little Kop):

 

 

Looking up from below and about a kilometre away (the closest I could get on dry land):

 

 

I've had various ideas about its formation. Perhaps it started off as a cave just below the top that eroded through the roof to form an oculus, which then lost one side of the bridge through further erosion. Then there's also case hardening in sandstone, forming an outer grey crust that's harder and more resistant to erosion than the yellow rock inside. This is more evident at Little Kop where there's clearly a cap above the honeycombed rock underneath. Another possible contributor is water flow, given the curved surface on the rock below Big Kop, but there's not going to be much going down there as where the man is standing on the right of the photo is pretty much the highest point in the vicinity.

 

Looking at the view from below, there's some very obvious (and regular) layering in the rock, with the rock finger roughly in line with one of the layer boundaries. I don't know what signifcance (if any) that has, though.

 

If case hardening really is an important factor in the formations, there's an excellent example of that in a cave next to the track on the way up, with a hard grey surface on the outside and the softer yellow honeycombed rock inside:

 

 

But beyond that I'm scratching my head, unsure whether any of this is right or if there might be some other mechanism involved. Nor do I have any good idea for the tasks to ask the finders to perform and the questions to ask, particularly as I want to avoid having them climb out along or under the rock fingers as it's a long way down.

 

So I'm open to ideas, although I suspect it would be better to have someone with geological knowledge accompany me to the site so they can see it in the flesh and look in all the right places. If I do get a virtual reward in the draw this month, I'll probably go down that path and just focus on the aesthetics of the site (including the awesome views), but it would still be nice to know how these formations came to be.

[Neos2]

I'm not sure I can offer much help. I found lots of sites that mention the carious layers of rocks in the park and their ages, which I am sure you have already found. But, correct me if I am wrong, you just want to be sure of how the Spion Kop formation was formed? 

https://hikingtheworld.blog/2022/02/12/spion-kop-brisbane-water/ says the formation was created by wind erosion. 

Quote

"From here it’s not far to Spion Kop at the end of the ridge, a vantage point with spectacular views over Woy Woy Bay, and all the way to the Pacific Ocean. Adding to the view is an unusual rock formation, where the wind has sculpted the rock into half of an arch. About 140m in height, Spion Kop (Afrikaans for “Spy Head”) was named after a famous Boer War battle site in the northern Natal Province – the hill is said to bear a close resemblance to its South African counterpart when viewed from a distance."

Have you asked the park rangers what they know about the formation? I have had very good luck talking with people who work near features. I also try to contact local geology clubs or schools to pick the brains of geology professors. 

[+hzoi]

My guess is that this formation is the remnants of a natural arch that collapsed. If I recall correctly, this would be a jughandle arch; unfortunately, my work connection is slow, so I can't confirm.

 

Here's a great site that goes into the types of natural arches and bridges and how they form. I've used it in the past for earthcaches.

 

naturalarches.org/

 

The pockets visible below the top of the projecting stone appear to be tafoni or haloclasty (salt weathering).

[+barefootjeff]

Thank you both for your suggestions. I'm still scratching my head a bit though...

 

5 hours ago, Neos2 said:

https://hikingtheworld.blog/2022/02/12/spion-kop-brisbane-water/ says the formation was created by wind erosion. 

 

3 hours ago, hzoi said:

Here's a great site that goes into the types of natural arches and bridges and how they form. I've used it in the past for earthcaches.

 

naturalarches.org/

 

The naturalarches.org page says right at the start, "Wind is not a significant agent in natural arch formation. Wind does act to disperse the loose grains that result from microscopic erosion. Further, sandstorms can scour or polish already existing arches. However, wind never creates them." Caves and honeycombing are a common feature of the sandstone cliffs around here, but from my reading there appears to be ongoing controversy as to whether they're formed primarily by wind or salt spray. Perhaps both are involved.

 

After the 17th, when I know whether I've received a Virtual Reward or not, I intend to revisit the site and pay closer attention to the rock layering, case hardening and any signs that they might be remnants of collapsed arches (such as evidence of a fracture where the missing part of the arch once joined the rockface). Also thanks for the suggestion to ask the ranger, I will next time I'm in touch with her though she can be a bit hard to pin down sometimes.

 

Edit to add: This has at least given me an idea for the EarthCache name, should it eventuate: "Fallen Arches".

Edited January 3 by barefootjeff

[Neos2]

7 hours ago, barefootjeff said:

The naturalarches.org page says right at the start, "Wind is not a significant agent in natural arch formation. Wind does act to disperse the loose grains that result from microscopic erosion. Further, sandstorms can scour or polish already existing arches. However, wind never creates them." Caves and honeycombing are a common feature of the sandstone cliffs around here, but from my reading there appears to be ongoing controversy as to whether they're formed primarily by wind or salt spray. Perhaps both are involved.

 

Edit to add: This has at least given me an idea for the EarthCache name, should it eventuate: "Fallen Arches".

I suspect you are correct; I think it likely the wind primarily played its part by carrying away material that had been eroded through other weathering processes. 

I do like the name you have suggested. 

[BlueRajah]

these formations or intendations are commonly refered to as Tafoni. See here https://en.wikipedia.org/wiki/Tafoni

 

If there is not one nearby on the subject, it is not a difficult Earthcache to put together. 

 

[+barefootjeff]

2 hours ago, BlueRajah said:

these formations or intendations are commonly refered to as Tafoni. See here https://en.wikipedia.org/wiki/Tafoni

 

If there is not one nearby on the subject, it is not a difficult Earthcache to put together.

 

Thanks. My main interest in that cave is the contrast between the grey, hard and relatively uneroded outside surface and the yellow, soft and deeply eroded interior. My understanding is that this is a result of case hardening (https://en.wikipedia.org/wiki/Case_hardening_of_rocks) and, if so, I'm wondering if the same phenomenon plays an important role in the formation of the Spion Kop rock fingers. The smaller one in particular looks like that might be the case, with the case-hardened top layer protecting the softer rock directly underneath.

 

 

Tafoni (specifically honeycombing) is very commonplace in the sandstone cliffs and larger boulders around here (it makes great hiding places for physical caches), and there are other EarthCaches that deal specifically with that, such as GC7AK5B, GC4RTXY and GC5FQAQ.

Edited January 4 by barefootjeff

[Johannis10]

The location is very impressive.

Instead of an earth cache, I could also imagine a multicache. For example, there is something to measure on the rock and it is just a qta stage. The final is then outside the park.

Greetings Johannis10

[+barefootjeff]

7 minutes ago, Johannis10 said:

The location is very impressive.

Instead of an earth cache, I could also imagine a multicache. For example, there is something to measure on the rock and it is just a qta stage. The final is then outside the park.

Greetings Johannis10

 

Thanks for the suggestion. I've done that with other caches inside national parks, most notably my Chasing Waterfalls series, but this spot doesn't really lend itself well to a multi as the entire 5km hike from the parking area (itself within the park boundary) to the headland is inside the national park and the only nearby spots on public land outside the park boundary already have physical caches there.

 

Yes, I know there's no distance restriction on a multi final relative to its listed coordinates and virtual waypoints, but I like to keep the final as an integral part of the whole experience. For my waterfall caches, I've generally placed the final along the same watercourse close to where it enters or leaves the national park. I don't want to have the challenging hike out to the spectacular headland topped off by a mediocre roadside final or have it a long drive off to somewhere unrelated to the rest of the experience.

 

At one point I considered placing an AL there and even set out along the walk to identify locations for it where reasonable questions could be asked, but soon realised that, once off the fire trails, none of the walking tracks around there are shown on the AL map. It's pretty wild country in there and wandering blindly without a map would be a good way to get lost. Phone coverage is also pretty marginal once you drop below the ridge tops.

 

My first preference for the site is still a virtual, and I'll find out on that score in a couple of weeks, but in the likelihood that I miss out again I think a reasonable EarthCache could be created, particularly if some of the other features along the way, like that cave, could be worked into the main geology lesson. As soon as we get a cool enough day, I want to head back out there with fresh eyes and might even discover enough along the way to weave it all into neat EC to make that my first choice and, if I do get a virtual reward, deploy that elsewhere.

[+CAVinoGal]

On 1/2/2024 at 2:36 PM, hzoi said:

The pockets visible below the top of the projecting stone appear to be tafoni...

 

On 1/3/2024 at 2:47 PM, BlueRajah said:

these formations or intendations are commonly refered to as Tafoni.

 

Until reading this thread, I had never heard of Tafoni.  Then, yesterday, I was researching Earthcaches in Cabo San Lucas in preparation for a visit there in February, and found this:

 

https://coord.info/GC7HT6P - Lands End Tafoni - there's a lot of information on the cache page, and I'm not sure if it clarifies or confuses things!

[+barefootjeff]

On 1/3/2024 at 9:36 AM, hzoi said:

My guess is that this formation is the remnants of a natural arch that collapsed.

 

I revisted the site last week and I'm pretty sure I've spotted where the missing half of the arch broke off from the rock wall.

 

 

 

There's also a large lump of rock on the left of the first photo that clearly broke off from the wall but didn't end up at the bottom of the cliff. It was perhaps part of the original cave ceiling that weathered through and progressively broke off.

 

I've written up drafts in Word for both a virtual and EC there and, depending on the outcome of the Virtual Rewards 4.0 draw on the 17th, will go ahead with one or the other. My draft virtual focuses on the views and attractions of the walk while the EC draft, of course, focuses on the rocks. Thanks for your help.

Edited January 14 by barefootjeff

[+hzoi]

On 1/13/2024 at 6:38 PM, barefootjeff said:

I've written up drafts in Word for both a virtual and EC there and, depending on the outcome of the Virtual Rewards 4.0 draw on the 17th, will go ahead with one or the other.

 

 

¿Porque no los dos?

[Neos2]

14 hours ago, hzoi said:

¿Porque no los dos?

Yes, why not? Why didn't I think of that!

[+barefootjeff]

14 hours ago, hzoi said:

 

 

¿Porque no los dos?

 

That thought did cross my mind, and perhaps sometime in the future I might do that, although it'd be better if someone else did (perhaps someone inspired to create an EC after completing the virtual). We'll see.